Being gay is just a coin flip that comes up heads every time. Straight is tails. And bisexuals? A more standard random flip distribution.
Discovering a therapy that helped change that coin flip to a different desired result misses the point entirely – that we are not who we fuck, or who we want to fuck.
All sexual identity and preference do are give promising hints at how much you are likely to enjoy an erotic encounter with each of your fellow 6950389928 space travellers (and says nothing about their reciprocated enjoyment, even when your self-perceived preferences are compatible).
Everything else – from powerlifting to showtunes to handbags to the LPGA – is really just a part of our desire for interpersonal resonance, non-familial community, and pure lazy fear of having to proactively express our own desires in a world that would be otherwise devoid of sexual assumption and pigeon-holing.
So, to get back to the original question – the initial reactions to such a therapy would basically be across the board as others have indicated, and generally not all that positive…
But after a generation or two? Imagine a typical US high school in which kids are mocked for being the offspring of a formerly gay father – by peers who are themselves the genetically-modified offspring of a lesbian couple… A society in which "passing" as gay, lesbian, bi, or straight has the same advantages and stigmas within industries and communities that used to (and in some cases, still) apply to things like race, religion, and cultural/ethnic background.
Simply put, any effort to apply the idea of scientific 'correctness' to societally-constructed norms around any form of human behavior is a hateful expression of intolerance, and a waste of time and money, period. What we really need is a therapy that replaces beliefs with questions – perhaps combined with an endorphin-rush at the search for, rather than the discovery of, any and all resultant answers.